1. In the present study we have classified the receptor(s) mediating increases in intracellular calcium concentration ([Ca2+]i) in human washed platelets and compared the pharmacological profile obtained with that observed in Jurkat cells, stably transfected with a bovine P2Y1-receptor. 2. The P2Y1-receptor antagonist, adenosine-3'-phosphate-5'-phosphate (A3P5P), competitively antagonized agonist responses in both Jurkat cells, and in platelets with similar affinities (pK(B) of 5.8 and 6.0, respectively). 3. The selective P2Y(ADP) antagonist, AR-C66096, exhibited partial agonism in the Jurkat cells with an affinity (pK(A)) of 4.9. This value is consistent with its known P2Y1-receptor activity. In platelets, AR-C66096 at a concentration (0.1 microM) approximately 100 fold greater than its known P2Y(ADP) receptor affinity, had no effect on ADP-induced increases in [Ca2+]i. 4. The ability of adenine nucleotide analogues to elevate [Ca2+]i in the Jurkat cells was also determined. The rank order of agonist potency (p[A]50) was: 2-MeSADP (8.3)>2-ClATP (7.8)>ADP (7.5)=2-MeSATP (7.4)>ATPgammaS (6.5)>ATP (6.2), with ATP appearing to be a partial agonist. 5. The same rank order of potency was observed when similar experiments were performed in platelets. However, the absolute potencies of all the agonists and the intrinsic activities of both ATPgammaS and ATP were lower in platelets. 6. The operational model of agonism was used to test whether the agonist concentration-effect profiles obtained in these two cell types could be explained on the basis of differences in receptor reserve. The analysis indicated that the data obtained in platelets closely resembled that predicted for a low density or poorly coupled P2Y1-receptor system. 7. The hypothesis that the observed partial agonist behaviour of ATP was the result of receptor activation by contaminating ADP with concomitant receptor blockade by ATP, was tested in the platelet system. This hypothesis was supported by a theoretical analysis, which yielded an affinity value for ATP similar to that obtained previously at P2Y1-receptors. 8. In summary, the results of this study indicate that human washed platelets contain P2Y1-receptors which mediate increases in [Ca2+]i and that this receptor population is pharmacologically distinct from P2Y(ADP)-receptors.